Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

environmental conditions and a high incidence of fungal diseases in greenhouses
[62]. Variations in field results are because of the heterogeneity of abiotic and biotic
factors and competition with indigenous microorganisms. A thorough knowledge of
these factors can help in determining the optimal concentrations, timing and placement
of inoculants and soil and crop management strategies to enhance survival and
proliferation of the inoculants [56]. Another problem associated with PGPR is their
low survival in the field or too high a concentration needed to exert the desired
activity. Like chemical pesticides, the practical use of PGPR as microbial fertilizers or
pesticides and their efficiency is strongly dose dependent [57]. Engineering the
rhizosphere by manipulating their host plant or their substrates or altering agronomic
practices is a better option today for enhancing the PGPR function and
properties of [56].
Other areas that need to be focused on in the field of PGPR are the development of
better formulations to ensure PGPR survival and activity in the field and compatibility
with chemical and biological seed treatments [61–64]. The toxicological risk
and the environmental impact associated with the introduction of PGPR into the
food chain or the environment are also a matter of concern.
10.6
Applications of PGPRs
PGPR can be used in a variety of ways when plant growth enhancement is required,
especially in agriculture, horticulture, forestry and environmental restoration.
10.6.1
Applications of PGPR in Agriculture
10.6 Applications of PGPRsj203
PGPR are most commonly used in agriculture. Up to 50–70% increase in crop yields
has been reported by researchers upon addition of PGPR. Plant growth benefits,
occurring upon PGPR addition, are as follows:
. increase in germination rates;
. increase in root growth;
. increase in yield including grain size, leaf area;
. increase in chlorophyll, magnesium, nitrogen and protein contents;
. increase in hydraulic activity, that is, fluid movement within the plant;
. tolerance to drought and low temperature;
. delayed leaf senescence; and
. disease resistance.
As the soil is an unpredictable environment, sometimes unexpected results are
observed owing to a low soil pH, high mean temperature and/or low rainfall during
the growing season. These undesirable conditions lead to low root colonization by
PGPR [65–67]. It was reported that climatic variability also plays a role in the
effectiveness of PGPR. Although field results vary, if a PGPR is found to be